The invention relates to an electric welding device and to a method of welding a fastening element, in particular a weld stud, on a workpiece.
Electrically operated welding devices are known which operate by the lift-and-strike method. The lift-and-strike method is a particular type of arc stud welding method in which the stud is applied to the workpiece and is removed from it to form an arc between the stud and the workpiece. After a molten pool has been formed on the stud and on the workpiece, the stud is re-applied to the workpiece while extinguishing the arc and is held in its position until the molten material solidifies. A device which operates by the lift-and-strike method is known, for example, from German Patent No. 32 01 979.
The supply of fastening elements to the welding device is substantially automated. The fastening elements are conveyed pneumatically from a store via a supply line to the welding device. The device has a guide member with a guide duct to which a fastening element is supplied. In the known devices, the conveying and holding system has, at its free end, several flexible holding tongues for grasping and holding a fastening element. A loading member capable of moving to and fro in the longitudinal direction of the guide duct is provided for positioning a fastening element in the conveying and holding system. It has a head which can be brought to rest on the fastening element. An electric welding device of this type is known from German Utility Model No. G 295 13 158.6.
Fastening elements are welded to a workpiece using the electrically operated welding device. The fastening elements are preferably substantially tubular in design. During the welding process there is a risk with tubular fastening elements that the molten material produced will be drawn radially inwardly owing to the magnetic field and constriction of the arc. Owing to this effect, there is a risk that the welded joint between the fastening element and the workpiece will be unsatisfactory.
A welding device is known from German Patent No. 42 00 199, in which a gas is supplied to the welding position. For this purpose, the device has a gas duct which is formed in a stud holding device. The stud has a through-bore which is formed substantially in the longitudinal direction of the duct and through which the gas passes to the welding position.
On the basis of the foregoing, the object of the present invention is to provide an electric welding device and a method of welding a fastening element, in particular a weld stud, by means of which the penetration of molten material formed during the welding process into the substantially tubular fastening element is avoided.
According to the invention, an electric welding device for welding a tubular fastening element, in particular a weld stud, on a workpiece with a conveying and holding system comprises a guide member with a guide duct to which a fastening element is supplied and at one of whose ends the fastening element is held during the welding process, and with a loading member which is capable of moving to and fro in the longitudinal direction of the guide duct and has a head which can be brought to rest on the fastening element is characterized in that the loading member has a duct with an inlet and an outlet, the outlet being formed at the head. In the guide duct there opens a gas supply duct which is closed by the loading member in a first position of the loading member and can be connected to the inlet in a second position of the loading member. The loading member adopts the second position during the welding process, so a gas can pass via the gas supply duct and through the loading member to the weld stud which has a through-duct. Admission of the melt during the welding process into the fastening element designed substantially as a hollow body is prevented by the supply of a gas to the welding position. With respect to the gas supply duct formed in the guide duct, the loading member acts as a valve which only clears the gas supply duct when the welding process is carried out.
A design of a welding device in which the guide duct and the loading member have a substantially circular cross section is preferred. The loading member can be a tubular component of which the end opposite the head is hermetically sealed.
According to a further advantageous embodiment of the welding device, it is proposed that the loading member be guided in a substantially gas-tight manner in the guide duct. This embodiment of the welding device is advantageous if the gas is an inert gas, in particular an argon-containing inert gas. The consumption of the gas is reduced by the gas-tight design so the economic viability of the welding device is further improved.
According to a further advantageous embodiment of the welding device, it is proposed that the inlet into the duct of the loading member be designed in the form of an elongated recess which extends in the longitudinal direction of the loading member. The length of the recess or of the inlet is calculated such that it corresponds at least to a displacement path of the loading member during the welding process. This ensures that the supply of gas is not interrupted during the advance of the loading member.
According to a further advantageous embodiment of the welding device, it is proposed that the loading member have a nozzle with the inlet, the nozzle penetrating at least partially into a socket formed on the guide member. The nozzle is preferably guided in a substantially gas-tight manner in the socket. With this embodiment, a reliable and gas-tight supply of a gas into the loading member is ensured using simple means.
The gas which is supplied in the device can be supplied from a gas source, for example gas bottles, via a feed line connected to the gas supply duct. The gas is preferably air.
To avoid oxidation in the welding position, as described in German Patent No. 42 00 199, it is proposed that the gas be an inert gas.
With the method according to the invention for welding a fastening element, in particular a weld stud, on a workpiece, it is proposed that a fastening element comprising a through-duct be supplied to a conveying and holding system comprising a guide member with a guide duct and be conveyed by a loading member capable of moving to and fro in the longitudinal direction of the guide duct to one end of the guide duct and be held therein during the welding process, the loading member closing, in a first position, a gas supply duct which opens into the guide duct and clearing it in a second position so a gas flows through a duct formed in the loading member and the through-duct in the fastening element to the welding position.